Lab talk

Optical nanosensors based on plasmonic nanostructures have great potential in chemical and biological sensing applications by taking advantage of the highly sensitive optical response of metallic nanostructures to changes in the local environment. Individual metallic nanoparticles and array structures have been studied extensively in the context of their refractive index sensing abilities. In general, the shift in surface plasmon (SP) resonance wavelength as a function of the refractive index of the environment is employed as the underlying sensing mechanism.

In a recent study, researchers from the Chinese University of Hong Kong (CUHK) have found that the sensing sensitivity of a hybrid SP-waveguide mode (SWG) sustained by one-dimensional gold gratings with deep grooves significantly exceeds that of pure SP modes (SSP) in shallow grooves. More importantly, the team found that while SSP is mainly controlled by the periodicity of the grating, the shape of the groove governs SWG. By properly designing the groove shape, such as its width and height, SWG can be increased up to 1500 nm/RIU.

The scientists have derived a simple but useful analytical expression to describe SWG, which not only reveals the origin of waveguide mode’s sensitivity, but also provides useful guidance for the theoretical design and experimental realization of high-sensitivity metallic grating-based biosensors.

The concept of employing a hybrid plasmon-waveguide mode has opened up a completely new perspective in improving the sensing sensitivity of plasmonic nanosensors. This hybridization concept can also be applied to tune and optimize other parameters such as the spectral contrast and the resonance linewidth, which are also of critical importance to increase the overall sensing figure of merit for a general class of plasmonic nanostructures including nanoparticle clusters and hybrid metallodielectric nanostructures.

About the author

Dr Dang-Yuan Lei was a previous member of the research team at CUHK led by Prof. Hock-Chun Ong and is now a postdoctoral research associate in the nanophotonics group led by Prof. Stefan Maier at Imperial College London. His research interests centre on the theoretical design and experimental investigation of novel plasmonic nanocavities and their applications in biosensing, photocatalysis, light-emitting devices and nanomaterial studies. Prof. Hock-Chun Ong has been actively studying surface plasmons for many years, investigating the fundamentals of surface plasmons and developing next-generation solid-state lighting devices and biosensors.